Think of the black area around your windshield like trim on your house, it finishes it off. It also keeps the UV rays of the sun from breaking down the factory seal (urathane). The most interesting fact is

that 75% of windshields that crack get hit by rocks in this area.

Two reasons for this is 1, you never see a chip in this area unless you look really close. It is a good idea every now and then to focus in this area, if you see a chip, look close to see if it has a star or dark shadow under it. If it does, call your windshield repair professional.

2, alot of stress is put on the windshield the closer to the frame making it more breakable.